In response to receipt by a first coherency domain of a memory access request originating from a master in a second coherency domain and excluding from its scope a third coherency domain, coherence participants in the first coherency domain provide partial responses, and one of the coherence participants speculatively provides, to the master, data from a target memory block. The data includes a memory domain indicator indicating whether the memory block is cached, if at all, only within the first coherency domain. Based on the partial responses a combined response is generated representing a systemwide coherence response to the memory access request. In response to the combined response indicating success and the memory domain indicator indicating that a valid copy of the memory block may be cached outside the first coherence domain, the master discards the speculatively provided data and reissues the memory access request with a larger broadcast scope.